It is frequently important to measure or sense an angular displacement and/or relative torque between first and second shafts. The relative displacement may be measured by a small angle displacement sensor. The relative position may then be used to derive the torque applied between the two shafts.
For example, power steering systems in motor vehicles and the like are designed to provide appropriate hydraulic or electrical assist to allow a driver to complete a turn of the motor vehicle. The driver typically turns a steering wheel which is connected to a first shaft. The first shaft is coupled to a second shaft which is connected to a steering mechanism. The first and second shafts may be coupled by a compliant member, such as a torsion bar. Typically, the first shaft may rotate with respect to the second shaft by a predetermined number of degrees, e.g., +/−12 degrees. Mechanical stops may prevent further movement. The amount of assist is determined as a function of the amount of torque being applied to the first shaft.
One common sensor to detect position and/or torque uses a potentiometer-based approach. Two stiff disks, each carrying a resistive film trace on a surface are rigidly attached to the torsion bar, one at each end. The twist on the torsion bar resulting from torque being applied to the shaft is translated into an angular displacement between the two disks and reads as resistance value by means of spring loaded contacts sliding on the resistive traces.
Other types of torque sensors utilize one or more magnets for generating a magnetic field. The magnetic circuit typically includes a second magnetic structure which forms a gap. A sensing device, disposed within the gap, detects changes in the magnetic flux which is used as an indication of the relative displacement between the first and second shafts.
One such system is disclosed in US Patent Application 20040011138, published Jan. 22, 2004 (hereafter “Gandel”). The second magnetic structure in Gandel is made up of two ferromagnetic rings, each having a plurality of axially oriented teeth. Each ring includes a circular flux-closing zone, which is parallel to the flux-closing zone of the other ring. The teeth of the rings are generally perpendicular to the flux-closing zones and are interleaved.
One inherent problem with the Gandel device is that it is sensitive to mechanical misalignment during assembly. Specifically, the axial teeth of the rings require very accurate placement with respect to each other. A deviation in the relative position of the rings and teeth with respect to each other will cause reduced performance of the device. It is difficult to accurately align the teeth of the rings and to maintain their relative position to maintain the correct distance from tooth to tooth.
Another disadvantage of the Gandel device is that it is sensitive to mechanical variation during operation. The device is sensitive to angular and parallel changes in the relationship of the two rotors to one another. Mechanical variation in these two directions will cause variation in the output.
Another disadvantage of the Gandel device is an output variation over 360°. This variation is caused by the magnetic structure of the device and the measurement location of the magnetosensitive elements.
Another inherent problem with the rings of the Gandel device, and other sensors of this nature, is that they are complex and difficult and costly to manufacture.
The present invention is aimed at one or more of the problems identified above.